Presently, receivers for differential signals are implemented in integrated circuitry using analog circuits. FIG. 1 depicts, for example, a system diagram of a typical prior art transmitter device 11 coupled to a link channel 13 for sending differential signals to a receiver 15. The receiver may be implemented as a receiver and analog front end (RXAFE) in an integrated circuit, or on a circuit board, or in multiple integrated circuits. The receiver extracts the received data from the analog differential signals and after some processing, received digital data signals may be output by the receiver. The data may be transmitted over a SerDes link using self clocking techniques as is known in the art.
When an analog receiver circuit is part of an integrated circuit, changing the parameters of the analog components used in the receiver becomes difficult. Some known prior art approaches employ external components to compensate the frequency response of the receiver for different channels and transmitters. Some known approaches may include trimming of on-chip resistors and the like. None of these trimming or tuning approaches is adaptive to later changes in the system. That is, if the receiver integrated circuit is later coupled to a different channel link or to a different transmitter, or to a channel that has time varying characteristics, the prior art integrated receiver may then be optimized for the wrong environment.
FIG. 2(A) depicts an example frequency response curve for the channel link. As is known to those skilled in the art, signal losses occur in a transmission channel which results in a gain loss (magnitude falls) in the signal, especially above certain frequencies. These losses may be due to several factors, including without limitation skin effects, dielectric coupling losses, and other transmission losses. The total channel loss appears similar to the frequency response of a low pass filter. In order to compensate the received signals for losses in the channel, the receiver should act as a high pass filter (in terms of frequency response). FIG. 2(B) depicts the desired frequency response of a receiver coupled to the channel. By providing gain at the correct frequencies, the receiver can compensate the signals received for the channel losses and restore them to the original magnitude at the transmitter. Thus, a desired receiver frequency response is matched to and compensates the channel losses in the channel frequency response. The desired total frequency response of the channel and the receiver is one of flat loss and bandwidth extension.
FIG. 3 depicts an example combined frequency response for the channel response in FIG. 2(A) and the receiver response in FIG. 2(B). The magnitude stays constant across the frequency range (flat loss) and the channel loss effect has been compensated, maintaining the signal magnitude at the higher frequencies where channel loss would otherwise occur (bandwidth extension).
The receiver frequency response is dictated by the linear equalizer function (LEQ) within the receiver analog front end (RXAFE) circuit. The LEQ should ideally be matched to the channel frequency response. This concept is illustrated by the curves in FIG. 4. In the top graph illustrating three curves labeled “channel 1”, “channel 2” and “channel 3”, the frequency responses for three different channels that the receiver circuit and analog front end circuitry RXAFE could be coupled to are shown. Each channel has different channel loss characteristics. In the bottom graph, the receiver LEQ frequency responses needed to compensate each of these three channels is shown. If the receiver is to operate with a variety of channel conditions, the LEQ should be adaptive so as to compensate for different loss characteristics in the channel or for different channels.
An LEQ frequency response has at least 3 important components, as illustrated by the response curve in FIG. 5. The gain peak frequency Fgp is one objective characteristic and represents the frequency where the most receiver gain is obtained In order to compensate for channel loss, this should correlate to the frequency that causes channel loss in the channel frequency response. The Gmax characteristic indicates the maximum gain needed in the receiver including the LEQ. The deltaG characteristic is the difference in gain needed between the high and low frequency receiver responses. Typically at lower frequency, the channel losses are not significant and thus the receiver does not need to have gain for those portions of the received signal.
A prior art approach to providing a receiver function is shown in FIG. 6. The channel 13 is depicted coupled to a receiver function 60. This receiver function may be one or more integrated circuits on a circuit board, discrete circuitry, or increasingly the receiver function may be part of an ASIC or SOC integrated circuit that includes other functions such as signal processors and the like. The receiver front end 63 includes an analog receiver and linear equalizer function as is known in the prior art. The analog signals are then transmitted in pairs to the clock and data recovery block 65 which includes, without limitation, an analog to digital converter, clock recovery circuitry. The recovered serial data stream is then converted into parallel data words by the series input parallel output circuit (SIPO) 67, which may be implemented as a FIFO, circular buffer, shift register, or other digital logic circuit known in the art.
The inputs to the receiver front end 63 are labeled (1), (2) and (3) and represent an input for altering the three characteristics described above, that is, the peak gain frequency Fgp, the maximum signal gain Gmax, and the gain difference deltaG. These inputs must be provided by external circuitry or additional internal circuitry. These inputs in the prior art are chosen with respect to the channel frequency response through a tuning or calibration procedure, for example, and are fixed. If the channel response varies with temperature, or other variations occur, or if the receiver device is coupled to a different channel, the circuitry driving these inputs should be changed, otherwise the response of the receiver LEQ is no longer able to compensate for the channel losses.
The increasing trend of the use of integrated circuitry in forming analog receivers for serial differential channel communications increases the need for receivers that adaptively adjust to provide the correct compensation.
Thus, there is a continuing need for improved methods and circuits to address these and other problems with the receivers of the prior art.